A push-push voltage-controlled oscillator (VCO) generates a fundamental oscillation signal, and a second harmonic oscillation signal that has a frequency which is twice a frequency of the fundamental oscillation signal. A conventional frequency doubler may be coupled to the push-push VCO for generating, based on the second harmonic oscillation signal, a fourth harmonic oscillation signal that has a frequency which is twice the frequency of the second harmonic oscillation signal. However, when operating in rather high frequency, the conventional frequency doubler has rather high signal attenuation, which renders the fourth harmonic oscillation signal too weak to be used.